In recent years, aluminum nitride single crystals have been noticed particularly from the viewpoint of applying them to substrates for semiconductor devices or the like. As the process for producing the aluminum nitride single crystals, it is known that a process comprises sublimating an aluminum nitride starting material, depositing an aluminum nitride single crystal on a seed crystal such as silicon carbide, aluminum nitride or the like and thereby growing the aluminum nitride single crystal.
For example, Patent document 1 discloses an experiment such that a sintered mass obtainable by sintering 99.99% powdery aluminum nitride is used as a starting material for sublimation and a susceptor having a bevel surface and a seed crystal are used and thereby a single crystal having a large diameter can be prepared with a small amount of defects. It further discloses that the half width of a rocking curve is small and a single crystal having a thickness of about 20 mm can be prepared. Patent document 2 discloses an experiment such that powdery aluminum nitride is used as a starting material for sublimation, a single crystal is grown while contacting the single crystal with the inner wall of a crystal growing vessel and thereby polycrystallization is depressed. It also discloses that an aluminum nitride single crystal having a low surface roughness, and having a diameter of 48 mm and a thickness of 1.5 mm can be obtained.
Meanwhile, for a process for producing a whiskery aluminum nitride single crystal, Patent document 3 discloses a process for producing whiskers of an aluminum nitride single crystal which process comprising using a composition obtainable by blending powdery alumina with a specific metal oxide as a starting material, heating the blend in a nitrogen atmosphere in the presence of carbon and thereby forming the whiskers in the composition.
Patent Document 1: JP-A-2004-284869
Patent Document 2: JP-A-2006-21964
Patent Document 3: JP-A-2005-132699
As described above, conventionally known processes for producing aluminum nitride single crystals have problems that the growing rate is slow and it needs a very long period of time for producing a large single crystal although they can prepare high quality aluminum nitride single crystals.
Under the circumstances, it is an object of the present invention to provide a process for producing an aluminum nitride single crystal having high quality in a high growing rate efficiently.
In the conventional process for producing a whiskery aluminum nitride single crystal, as described above, since the whisker formation is caused in the powdery starting material, there are problems such that it is difficult to achieve complete processing of all of the starting material powder, and the resulting whiskers contain the powdery starting material which is difficult to be removed. Furthermore, a metal oxide used as the starting material may also be present in the formed whiskers.
Under the circumstances, it is another object of the present invention to provide an aluminum nitride single crystal forming polygonal columns containing a slight amount of fine powder or impurities, and to provide a process for producing the aluminum nitride single crystal forming polygonal columns.